Meanwhile, and always, the supreme good of the City of God is everlasting and perfect peace and not merely a continuing peace which individually mortal men enter upon and leave by birth and death, but one in which individuals immortally abide, no longer subject to any species of adversity.Yet, this is not to say that there is no happiness in this life. Those who keep there eyes on the City of God and use the goods of this world to Its glorious end can be said to be happy as well, "though more in hope than in present happiness." However, the idea of being truly happy apart from having tied that happiness to the City of God is "an illusory happiness and, in fact, a great wretchedness, since it makes no use of the true goods of the soul." Just as "no wisdom is true wisdom unless it [along with prudence, fortitude, temperance, and justice]... is directed to that goal in which God is to be all in all in secure everlastingness and flawless peace." There is no true happiness any more than there is any true virtue outside of God.
If we accept Cicero's definition of a "republic" that he puts in the mouth of Scipio in his book On the Republic, "there never existed any such thing as a Roman Republic."
Cicero (well, "Scipio") argues that a commonwealth is 'the weal of the people.' If this is accurate, there was never any true Republic because there was never "any true weal of the people." Specifically, this means a body of citizens working together by acknowledging each other's rights and working towards the common good. This means that justice must define the state, for "where there is not true justice, there is no recognition of rights." And of course by "rights" and "justice" we don't mean any old law that gets passed by those in power--many of those can be based on self-interest and self-advantage, rather than springing from the "fountainhead of justice."
So, where there is no justice, there is no recognition of rights, and no citizen body, and so no commonwealth. So what about Rome? Well, "Justice is the virtue which accords to each and every man what is his due." But if we rip ourselves from God and worship demons, we have hardly given ourselves our own due.
In On the Republic, Cicero (like Plato's Republic, which Cicero is largely transcribing into Latin) gives first a defense of injustice and then defends justice. The defense he gives is that God must master man, and the soul master the body. But "what fragment of justice can there be in a man who is not subject to God"? And if this man has no justice, "then there is certainly no justice... in an assembly made up of such men." And so there can be no commonwealth, where there is no justice. There can be no true justice for those who do not worship the true God, and so there can be no true commonwealth for a body of such people.
And our God is the true God to be worshiped and adored, not the pagan false gods.